Sulfur-based substrates

One reason for the low yields in these reactions was eventually traced to the attack by butyllithium at the sulfur center in competition with deprotonation of the terminal acetylenes. By using the more hindered mesityllithium, we were able to improve the yield of the 24-mem-bered ring, mixed heterocyclic 95 from 5 ti (butyllithium as base) to 24% (mesityllithium as base) [22]. The earlier reactions outlined in Fig. 9-26 that involve deprotonation of sulfur-containing substrates by butyllithium have not been reexamined using mesityllithium as the base. 

Table 2.8 compares hydrolysis half-lives with half-lives for reaction with sulfur-based nucleophiles for several halogenated aliphatics. These data show that the environmental half-lives for substrates such as 1-bromohexane and 1,2-dibromoethane can be substantially reduced in the presence of HS and polysulfides. Enhanced degradation of 2-bromopropane and 1,1,1-trichloroethane, as well as chloroform and carbon tetrachloride (results not shown) was not observed, suggesting that steric hindrance significantly impedes reaction with the sulfur based nucleophiles (Haag and Mill, 1988a). 

As with other rearrangements, the Payne rearrangement also has its variants. Similar transformations of substrates with nitrogen- or sulfur-based functionalities at the carbon atom adjacent to the epoxide are known as aza- or thia-Payne rearrangements, respectively, and there exist a nurnber of noteworthy previously reported examples of each. Although study of these systems has been somewhat limited in recent years, they are briefly surveyed in the following sections. 

The Wenker aziridine synthesis entails the treatment of a P-amino alcohol 1 with sulfuric acid to give P-aminoethyl sulfate ester 2 which is subsequently treated with base to afford aziridine 3. Before the discovery of the Mitsunobu reaction, wbicb transforms an amino alcohol into an aziridine in one step under very mild conditions, the Wenker reaction was one of the most convenient methods for aziridine synthesis. However, due to the involvement of strong acid and then strong base, its utility has been limited to substrates without labile functionalities. 

Amides are very weak nucleophiles, far too weak to attack alkyl halides, so they must first be converted to their conjugate bases. By this method, unsubstituted amides can be converted to N-substituted, or N-substituted to N,N-disubstituted, amides. Esters of sulfuric or sulfonic acids can also be substrates. Tertiary substrates give elimination. O-Alkylation is at times a side reaction. Both amides and sulfonamides have been alkylated under phase-transfer conditions. Lactams can be alkylated using similar procedures. Ethyl pyroglutamate (5-carboethoxy 2-pyrrolidinone) and related lactams were converted to N-alkyl derivatives via treatment with NaH (short contact time) followed by addition of the halide. 2-Pyrrolidinone derivatives can be alkylated using a similar procedure. Lactams can be reductively alkylated using aldehydes under catalytic hydrogenation... 

The formation of colloidal sulfur occurring in the aqueous, either alkaline or acidic, solutions comprises a serious drawback for the deposits quality. Saloniemi et al. [206] attempted to circumvent this problem and to avoid also the use of a lead substrate needed in the case of anodic formation, by devising a cyclic electrochemical technique including alternate cathodic and anodic reactions. Their method was based on fast cycling of the substrate (TO/glass) potential in an alkaline (pH 8.5) solution of sodium sulfide, Pb(II), and EDTA, between two values with a symmetric triangle wave shape. At cathodic potentials, Pb(EDTA)2 reduced to Pb, and at anodic potentials Pb reoxidized and reacted with sulfide instead of EDTA or hydroxide ions. Films electrodeposited in the optimized potential region were stoichiometric and with a random polycrystalline RS structure. The authors noticed that cyclic deposition also occurs from an acidic solution, but the problem of colloidal sulfur formation remains. 

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